This invention relates to protectors of the general type used in central office telephone switching equipment. Devices of this type serve to protect the inside switching and like equipment from damage as a result of overvoltage and overcurrent conditions that may occur on the outside lines. Frequently these devices are referred to in the art as central office protectors.
One such type of protector is shown in U.S. Pat. No. 3,975,664 that issued Aug. 17, 1976. The protector shown and described in that patent provides a relatively fast operation on sensing and overvoltage or overcurrent condition on the line. A gas tube surge arrester is utilized to provide a discharge path to ground from each side of the line. In an overcurrent condition in the line a solder element is melted resulting in a spring-biased element being caused to engage the ground electrode of the gas tube so that a direct metallic path is provided from the line to ground. Furthermore, the device of the aforesaid patent is relatively small and compact, is simple and inexpensive to manufacture, and at the same time is capable of being mounted in a substantially standard base and housing. The device also has a five pin base construction which enables the unit to be plugged into known types of terminal blocks for central office protectors.
While a device of the foregoing type has proven to be satisfactory, a problem may exist if the gas tube within the device becomes vented to atmosphere. In such case protection would be lost because the electrodes of the gas tube would then be exposed to atmosphere. This would result in the voltage breakdown of the gas tube being far in excess of that suitable for line protection.
It will be appreciated that a gas tube surge arrester which has failed by reason of leakage will be difficult to detect simply because the line to which it is connected continues to operate properly. Accordingly, it is desirable to provide some type of air gap as a back up protection arrangement in the event of failure of the gas tube due to leakage. Line protectors having gas tube surge arresters and back up air gap secondary arresters are known, but in most cases the devices use numerous special parts which results in an increased cost of manufacture over standard central office protector devices.